1. Field of the Invention
The present invention relates to a method for continuously treating wastewater produced by paper converting operations such as the manufacture of paper bags and paper boxes, including corrugated cardboard containers and the like. The invention specifically relates to a method for separately recovering in a continuous fashion a clarified water stream suitable for discharge into a municipal sewage system and a sludge from the wastewater produced during the operation of a paper converting plant and for treating the sludge to yield a stabilized solid safe for disposal by land fill.
2. Description of Related Art
In the course of converting paper into finished products, such as when manufacturing paper bags and paper boxes including corrugated cardboard containers and the like, a variety of waste streams are generated. Two of the most significant and troublesome from the standpoint of disposal are the wastewater streams produced by the printing and gluing operations.
Generally, paper converting plants use some type of flexographic ink to print onto the paper product. Thus, organic binders and vehicles such as maleic or acrylic resins and glycols and inorganic pigments containing a variety of heavy metals such as lead, chromium, copper, cadmium, arsenic, zinc, nickel, barium and other toxic materials such as cyanide widely used in such inks generally are present in the effluent wash water produced when cleaning the printing equipment. These metals often are toxic to microorganisms in conventional sewage treatment plants. The make-up of the ink waste also undergoes dramatic changes, sometimes frequently, as the nature of the ink used for particular paper products is changed.
While a variety of adhesives may be encountered in paper converting wastewater, by far the most widely used is based on corn starch. Due to its high organic content, starch adhesive waste and the washdown water obtained from cleaning the equipment used to apply the starch-based adhesive, exhibit very high total suspended solids, and biological and chemical oxygen demands (TSS, BOD and COD), and often contain other additives, making them wholly unsuitable for direct disposal into sanitary sewage systems.
In some paper conversion plants, the ink wastes are segregated and treated separately from the starch adhesive wastes. In the past, because of the variable nature of the ink waste these systems have been operated in a batch type manner. For example, the ink waste has been treated with inorganic agents such as ferrous sulfate and lime or sodium hydrochlorite and alum to flocculate and settle the heavy metals. Because the metals in the sludge are mobile, however, the sludge must be handled as a hazardous (toxic) waste. The starch adhesive waste has been enzymatically treated to render the wastewater more amenable to treatment by biological processes, such as activated sludge systems.
For simplicity and economy the waste streams produced in paper converting plants, such as during the manufacturing of paper bags and paper boxes including corrugated containers, are desirably consolidated and treated in one operation. This, however, complicates the treatment and ultimate disposal of the waste material. In particular, due to the presence of the ink waste, the sludge recovered from such combined wastewater stream has proven to be very difficult to dewater.
In U.S. Pat. No. 3,868,320 the treatment of a combined stream of ink waste and starch waste from a paper box manufacturing plant is described using a multivalent metal compounds such as slaked lime, calcium chloride, magnesium oxide, alum, ferrous sulfate or magnesium hydroxide as a flocculation agent, followed by settling, filtering or centrifuging. Because of the variability in the make-up of the wastewater stream, treatment is carried out in a batchwise manner. Only in this way, can the proper balance of the multivalent metal compounds used in this system be determined in order to yield a properly clarified water stream. Attempts at using such materials for continuous treatment proved unsatisfactory. Unfortunately, even batchwise treatment is not a total solution. Due to the hygroscopic nature of the resulting sludge, it is virtually impossible to dewater completely and very difficult to handle. Furthermore, the heavy metals content of the sludge recovered from simple dewatering is readily leachable, thus making the sludge unsuitable for landfill disposal and requiring it to be handled as a hazardous (toxic) waste. Thus, this prior art method simply has substituted one disposal problem for another.